To improve accuracy and enable ease in output interpretation, weighing scales have been developed using electronic circuitry that receives an analog signal generated by a weight-responsive transducer and converts the analog signal to a corresponding digital signal. The digital is applied in decimal format to a display such as light emitting diode or liquid crystal array. In one type of scale used in portion weighing, pushbutton tare is provided; with any tare weight on the scale, the display can be reset to zero and thereafter reads only weight added or removed. A resettable up/down counter is controlled by a dual slope integrator to successively up count and down count pulses generated by a clock source. The integrator in turn successively up integrates for a fixed time toward the magnitude of the voltage sample and down integrates at a fixed slope toward zero. At zero crossover of the down integrate signal, the counter output, which is proportional to input voltage, is latched for display. The count is reset to zero by operation of a tare switch and thereafter, accumulated count is proportional to change in applied weight, i.e., weight added or weight removed.
Weight to voltage transducers such as strain gauges typically used in electronic weighing scales generate a low level voltage that is linear with applied force within a voltage range on the order of two or three millivolts per 0.1 ounce of applied weight. Low frequency signal-to-noise ratio of analog to digital conversion circuitry operative with applied signals within that range is limited by factors such as thermal drift, slowly changing electrical and mechanical bias and varying environmental conditions such as air currents, temperature and humidity changes. As a result of this low level, low frequency noise, the reading of the digital display typically will drift from zero to indicate an error of one or more digits in each direction. For example, whereas a typical four digit display reading with no applied weight on the scale or with an applied weight at tare would be 000.0, the display in practice may read +000.1 or more in presence of normal drift. Although this output error is relatively small, a user in a commercial environment may consider any non-zero reading with no applied weight or at tare to be intolerable.
One object of the invention, therefore, is to provide automatic zero tracking in an analog to digital conversion system.
Another object is to provide automatic zero tracking in an electronic weighing scale incorporating an analog to digital converter system to develop a digital output reading proportional to applied weight.
Another object is to provide an automatic zero tracking circuit in an analog-to-digital conversion system employing a dual slope integrating type analog-to-digital converter driving a resettable up/down counter that accumulates counts proportional to input voltage magnitude.
Another object is to provide an electronic weighing scale wherein the display output is maintained at zero independently of drift and other low level noise.
Another object is to provide an electronic weighing scale of the type employing a dual slope integrating type analog-to-digital converter driving a resettable up/down counter wherein drift and other low level noise are automatically compensated to provide a continuous zero output display with no applied weight or at tare.
Another object is to provide an electronic weighing scale having an automatic zero tracking circuit to display a zero output with no applied weight or at tare independently of drift or other low level noise and wherein the zero tracking circuit distinguishes between drift and change of applied weight.